This proposal is concerned with the elucidation of the structure of the high affinity lgE Fc receptor from mast cells (FceRl). This receptor binds lgE antibodies and initiates cellular responces associated with allergic reactions and anaphylactic shock. We have succeded in growing crystals of a soluble fragment of the receptor using the vapor diffusion method of crystalization. This fragment consists of two immunoglobulin domains that carry the full binding affinity for lgE molecules. The largest crystals grow with dimensions of 80x80x300 microns. These crystals diffract X-rays to a resolution of 4.3 A, using an Elliot GX-13 rotating anode X-ray source and an exposure times of many hours. The current data indicate that the crystals belong to a tetragonal space group with approximate cell dimensions of 75x150x150 A. The size of these crystals and the weak diffraction observed using labortoey X-ray sources makes the use of a synchrotron X-ray source imperative.